String Theory versus Loop Quantum Gravity

Loop quantum gravity makes some definite predictions, which may mean that it could be tested well before string theory can be. As string theory’s popularity is being brought into question, the amount of research into LQG may end up growing.

Gravity exists (Duh!)

Oddly enough, because LQG was born out of general relativity, one question has been whether science can get general relativity back out of the theory. In other words, can scientists use loop quantum gravity to actually match Einstein’s classical theory of gravity on large scales? The answer is: yes, in some special cases (as does string theory).

For example, work by Carlo Rovelli and his colleagues has shown that LQG contains gravitons, at least in the low-energy version of the theory, and also that two masses placed into the theory will attract each other in accord with Newton’s law of gravity. Further theoretical work is needed to get solid correlations between LQG and general relativity.

Black holes contain only so much space

Loop quantum gravity’s major success has been in matching the Bekenstein prediction of black hole entropy as well as the Hawking radiation predictions. String theory has been able to make some predictions about special types of black holes, which is also consistent with the Bekenstein-Hawking theories.

So, at the very least, if scientists are able to create miniature black holes in the Large Hadron Collider and observe Hawking radiation, then it would certainly not rule out either of the theories.

However, the picture given by LQG is very different from that of classical black holes. Instead of an infinite singularity, the quantum rules say there’s only so much space inside of the black hole. Some LQG theorists hope they can predict tiny adjustments to Hawking’s theory that, if experimentally proven true, would support LQG above string theory.

One prediction is that instead of a singularity, the matter falling into a black hole begins expanding into another region of space-time, consistent with some earlier predictions by Bryce DeWitt and John Archibald Wheeler. In fact, singularities at the big bang are also eliminated, providing another possible eternal universe model.

Gamma ray burst radiation travels at different speeds

Many of the experiments which could test whether the speed of light varies would also be consistent with loop quantum gravity. For example, it’s possible that gamma ray burst radiation doesn’t all travel at the same speed, like classical relativity predicts. As the radiation passes through the spin network of quantized space, the high-energy gamma rays would travel slightly slower than the low-energy gamma rays.

Again, these effects would be magnified over the vast distances traveled to possibly be observed by the Fermi telescope.